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Application of novel polymeric surface remediation technique based on flying jet plasma torch Wameath S. Abdul-Majeed, Ibtisam M. AL-Handhali, Shima H. AL-Yaquobi, and Khamis O. Al-Riyami Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02729 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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Industrial & Engineering Chemistry Research

Application of novel polymeric surface remediation technique based on flying jet plasma torch Wameath S. Abdul-Majeed a,*, Ibtisam M. AL-Handhali a, Shima H. AL-Yaquobi a, Khamis O. Al-Riyami a

b

Department of Chemical and Petrochemical Engineering – University of Nizwa – Oman, PC

616 b

Daris Centre for Scientific Research and Technological Development – University of Nizwa

- Oman, PC 616 *

Corresponding author email: [email protected]

Abstract A flying jet plasma torch (FJPT) was applied to treat raw polymers before end use process. Specimens of the raw polymers (Polypropylene, Polystyrene, and two grades of Polyethylene) were treated for few minutes. The examined specimens were analyzed by using SEM, AFM and EDS. The results indicated that FJTP effectively stimulated the topographical modifications and changed the chemical composition of the surface, which consequently affected the roughness of the surface. Machinability of the examined high density polyethylene (HDPE) was investigated through measuring the melt mass flow rate index (MFI). The results showed that MFI has increased around 30% after 5 minutes treatments. Longer treatment periods has resulted in reductions of MFI and attributed to the thermal effects. Measured MFI for treated HDPE slightly decreased along 3 days after treatment. Hence, FJPT proved useful for treating polymer surface homogeneously and tailoring the surface chemistry for the required end use. Keywords: Polymeric surface treatment; Non thermal plasma; Adhesion characteristics

Introduction Because of superior mechanical, thermal, and electrical properties, polymeric materials are constantly replacing fabrication materials that are commonly used in different industries 1-3. In this respect, polymers should exhibit good adhesion properties in order to replace steel or aluminium effectively 4. Though some polymers are distinguished with acceptable adhesion properties, the majority are poor due to low surface energy, chemical inertness and smooth surface 5. Hence, polymeric film commonly needs some additional treatment to ameliorate the surface activity and improve wettability and adhesion factors 6. Some common techniques used for 1 ACS Paragon Plus Environment


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polymeric surface treatment were based on chemical methods (e,g. acid treatment) or physical methods (e.g. exposure to flame, ion beams, surface grafting, vacuum and atmospheric plasmas) 7-13. Those techniques were aimed at enhancing adhesion properties through improving chemical interactions; mechanical interlocking; internal diffusion of chains; and other weak inter-atomic forces. Among the polymeric surface treatment techniques, vacuum and atmospheric plasma were investigated intensively by different research groups and showed remarkable advantages such as surface cleaning and surface-chemical structure modification 14. Inducing plasma on the polymeric surface layer result in changing the chemical and physical properties due to integrated effects of oxidation, degradation, cross-linking and structural changes. In other words, applying plasma treatment for the surface polymer is aimed at incorporation of polar groups (e.g. C=O, OH and COOH) on the surface; thereby promoting the chemical interactions via increasing the surface oxygen content and the amount of mechanical interlocking 15. The roughness of the surface could also be affected by the plasma treatment leading to changes in the surface wettability. Some research groups worked on elucidating the kinetics of polymer treatment via non thermal plasma. A mechanism based on free-radical degradation of polyolefins was proposed in references 8, 16-17. It was reported that an attack occurs primarily at the tertiary carbon of the polyolefin chain leads to formation of other radicals and intermediate species. Based on the information given in the literature, a free radical reaction pathway is proposed as follows:

→ ∎ + ∎

→ ∎ + ∎

∎ + → ∎ ( )

∎ + → ∎ + (ℎ ) E, refers to a source of energy capable of bond breakage such as VUV photons, electrons or energetic ions. The formed peroxy intermediates may lead to crosslinking and functionalization of the polymer chain. in summary:

∎ + → ∎ → → Its noteworthy that plasma treatment lead to strengthening hydrophilicity of treated polymer surfaces, in usual. However, an increase in hydrophilicity is often lost over time. This phenomenon was called hydrophobic recovery and attributed to several reasons 18-27: (i) mobility of polymer chains which lead to re-arrangement of chemical groups on the polymeric surface; (ii) diffusion of low molecular mass oxidized species from outer layers into the bulk of the polymer; (iii) migration of additives introduced into the polymer from its bulk towards its surface (iv) further oxidation and degradation reactions at the plasma treated surfaces upon exposure to air. Hydrophobic recovery phenomenon was considered crucial and some researches 2 ACS Paragon Plus Environment

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worked on finding a way to suppress the effect. In this sense, Bormashenko et al. 24 reported that immersion of plasma treated polymeric material into high polarity liquids slows markedly the hydrophobic recovery. However, hydrophobicity of plasma irradiated films was restored up to such extent (incomplete) upon decreasing the temperature of the environment. By comparing the techniques of nonequilibrium plasma-surface modification of polymers (vacuum vs. atmospheric pressure), it has been shown that considerable similarities exist between the two processes. However, atmospheric non thermal plasmas (NTP) does not have a significant vacuum-UV component which were proven as key activators for the surface-modification reactions 17. Nonetheless, atmospheric plasma technique imparts an increase in the adhesive strength for plasma treated samples which correlates with increase in both surface energy and surface oxygen content 3. Moreover, atmospheric plasma was found more applicable and easier to implement as no need for vacuum tool exists. Hence, voluminous literature 27, 11-17 have investigated the application of NTP for different polymers surface modification by adopting dielectric barrier discharge (DBD) in both vacuum and atmospheric modes. However, the mentioned DBD plasma reactors were dedicated for laboratory scale work and in most cases only few millimetres thick of the examined polymers were put in plasma bulk (i.e. in the gap between the DBD reactor electrodes), which makes this concept difficult to apply for large scale applications. Other research endeavours were dedicated to process polymers in the particle form (powder) by using different kinds of plasma reactors such as fluidized bed reactors, downer reactors, batch reactors and barrel reactor 28-31. All the above mentioned plasma reactors relies on the DBD concept, and among of them the barrel reactor 27 was reported with remarkable efficiency as the polymer powder is rotated while its being subjected to the plasma effect. Nevertheless, it’s still difficult to apply those designs for industrial scale due to the size limitation (Paschen’s law) and high consumptions of power and gas. Alternatively, a flying torch which can deliver jet flow plasma on the polymer surface would become more practical method, functioning smartly for industrial scale. An important feature of plasma jet is the creation of plasma plumes in open space while providing a significant number of active species, such as radicals, electrons, and ions. which makes it as a unique tool for direct treatment of different kinds of materials 32. In this research, its hypothesized that the polymer raw material, prior to the end use, would respond more effectively to the plasma treatment. Hence, this study is dedicated to test a flying jet plasma torch FJPT 33 for treatment of different raw polymers prior to the end use. Our objective is to contemplate on the variances occur to the polymer surface after being subjected to the plasma treatment and elucidate the reflections on the end use process. Also, the hydrophobic recovery effect was measured for the treated specimens in order to evaluate the efficiency of the proposed technique. To the best of our knowledge, the developed polymeric treatment technique (i.e. FJPT) is the first mentioned in the literature.

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Materials and Methods In this research, a developed DBD platform - flying jet plasma torch (FJPT) was used to treat raw polymeric materials for several durations. The full details of the developed DBD platform - FJPT were presented in our previous work 33. The FJPT was powered by a DBD plasma power source (DIDRIV10), purchased from Information Unlimited/USA. The device is equipped with independent voltage control from zero to a maximum output of 40 kV, independent current control from 5 % to maximum output and independent frequency control from 20 to 70 kHz. Argon gas (99.995 %), purchased from Muscat gases Co./Oman, at a rate 100 ml/min was applied for flying jet plasma generation through Teflon flow meter type purchased from maxtec. Raw polymeric granules of: (1) Polypropylene (grade: SABIC® PP/ 108MF10 – Saudi Arabia) , (2) Polystyrene (standard grade: SABIC®EPS/452 – Saudi Arabia), (3) Linear low density Polyethylene (grade: SABIC®LLDPE/118LJ – Saudi Arabia), (4) Black high density polyethylene for pressure pipe (Borstar®HE3490-LS, Singapore) were treated in this study for several durations via FJPT. Figure 1 illustrates the granules of black HDPE pressure pipe grade under the influence of FJPT. The granules of the examined polymeric specimen were put in a matrix form inside the glass container in order to ensure uniform surface treatment. Five granules of each treated polymeric specimen, directly subjected to the plasma torch (the core of the granules matrix), were used for SEM, AFM and EDS analysis. The treated and untreated polymeric specimens were analyzed through scanning electronic microscope (SEM), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS) to elucidate changes in both surface topography and chemistry. SEM analyses were conducted through a device manufactured by JEOL (model: JSM-6510LA). The specimens were prepared for analysis by using a sputter coater manufactured by JEOL (model: JFC 1600). Sputter coating for SEM is the process of making a layer coating of electrically conducting metal onto poorly conducting metal. It enhances the imaging and reduces the damage which may caused by the high temperature inside the instrument. The conducting layer increases the signal of the secondary electrons required for the topographical test. In this study, specimens of the raw polymeric granules, before and after plasma treatment, were coated by using platinum (Pt) where a continuous Pt film with a grain size in the order of 2 nm was formed on the granule surface and counts as a good secondary electron emitter. As soon as the examined specimen is coated, it was put inside the empty column of the SEM device, and once the column is completely discharged from the air, the electronic gun releases a beam of electrons towards the sample surface ; also, some secondary electrons are released from the sample surface. These electrons are detected by a special detector and the final image is made up the number of electrons released from each point on the surface of the sample. Atomic force microscopy (AFM) has been shown as a powerful tool for visualising changes in the polymeric surface topography. In this study, AFM analyses were conducted via Veeco Innova System - USA, in which the images were acquired at a resonance frequency of 300 kHz (with 2 Hz scan rate and 1.2 µm scan size) and processed through Gwyddion software. The energy dispersive spectroscopy (EDS) has been used in this study to analyse the specimens constituent elements and their relative atomic and mass percentage. EDS analyses were conducted through a JEOL Analysis Station JED2300T, which is an integration system of TEM/EDS based on a concept of Image and Analysis. The melt mass flow rate index was measured for specimens of black high density poly ethylene (dedicated for pressure pipe manufacture), before and after 4 ACS Paragon Plus Environment

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plasma treatment, in order to contemplate on the enhancement towards the polymeric end use. The measurements were conducted through a melt flow index tester manufactured by Deepak Poly Plast Pvt. Ltd. at AL Hilal Polyethylene Pipe Industry – Nizwa. Four grams of HDPE samples, before and after plasma treatment, were fed to the device and heated up to 190oC. The measured MFI is expressed in grams of the extruded polymer per 10 minutes extrusion time.

Figure 1 Application of FJPT for HDPE treatment

Results and Discussion Characterization of the FJPT - power consumption and operating temperature It’s worth noting that the voltage lags the current by the phase angle in an alternative current power source. Hence, the consumption of electrical power in the plasma bulk does not correspond to the total power drawn from the mains. One valid approximation to estimate the power consumed in the plasma bulk is the voltage charge diagram (Lissajous figure) 34-35, in which the area enclosed by the Lissajous figure matches with the energy consumed per cycle of the applied voltage. Thus, the mean dissipated power in the plasma bulk is interpreted by the product of this energy and the applied frequency 36. In the DBD platform - FJPT used in this study, the area of the voltage-charge diagram was found to be 0.98 mJ 37. Hence, the power delivered to the DBD cell was estimated to be 29.4 watt (= 0.98 mJ × 30 kHz). Maintaining this power condition (around 2.5 kV at approximately 30 kHz), the argon plasma plume (plume length ≈ 0.5 cm flying outside the hose), shown in Figure 2, was found more uniform at an argon gas flow of 100 ml/min. It’s worth noting that the hot species temperature (outlet gas from the torch) was measured via lab thermocouple and found around 60oC. Accordingly, safe and reliable operation for the polymer treatment was attained through this configuration and condition.



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Figure 2 Plasma plume – FJPT

Optimization of plasma treatment duration As the examined polymers were encountered with thermal limits, the treatment duration was found as a critical mark that should not be exceeded. An optimization study was conducted in this sense, in which the specimens were subjected to FJPT effect for 1-5 minutes by applying the plasma conditions mentioned above. It was observed that exceeding 2 minutes treatment duration has resulted in endurable topographical changes but 6 minutes plasma subjection and onward led to gradual polymer melt. This was dually assured by testing the treated polymer performance through the melt flow index (MFI) test. Hence, examining four polymers by FJPT resulted in changes on the surface topography starting within 2 minutes and optimum curing at around 5 minutes.

SEM analysis The changes on the topography of the examined specimens, before and after plasma treatment, were explicated through analysis of scan electron microscopy shots. The images were taken at a voltage of 5 KV and a magnification ×1000 with a graphical scale = 10 . Images for polystyrene specimens before and after 2 minutes FJPT treatment are shown in Table 1. Images for other polymers are shown in Table S1 (supporting information). Clear changes in the polymeric surface topography, (i.e. reductions in the pores and pumps) are shown for polypropylene, polystyrene, linear low density polyethylene compared with lower changes occurred to the surface of high density polyethylene. These results proves the modification of the polymer surface morphology upon being subjected to plasma effect. The effects on the HDPE granule surface topography was found more clear for specimens treated for 5 minutes 6 ACS Paragon Plus Environment

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and considerable changes occurred in the case of 10 minutes treatment, as presented in Table 2.

Table 1. SEM images for polystyrene specimens before and after 2 minutes of FJPT treatment a- Polystyrene granule – before treatment

b- Polystyrene granule – treated for 2 minutes

Table 2 SEM images for HDPE specimens treated for 5 and 10 minutes a- HDPE granule - treated for 5 minutes

b- HDPE granule – treated for 10 minutes

AFM analysis Further topographic investigations were conducted via atomic force microscopy in which the results show that plasma treatment for 10 minutes resulted in rough morphology. Table 3 illustrates AFM images for HDPE before and after 5 and 10 minutes treatment duration.



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Table 3 AFM images for HDPE specimens before and after plasma treatment HDPE granule - untreated

HDPE granule – treated for 5 minutes

HDPE granule – treated for 10 minutes

According to the SEM and AFM results, it can be concluded that the surface roughness has increased gradually with the treatment time. This finding is in agreement with Pandiyaraj et al. 18 who reported an increase in the roughness of plasma treated polypropylene due to the removal of top few monolayers of the polymer films. Thus, the surface changed to more hydrophilic and the cross linking with liquid droplets became easy because of increased polar interaction. However, exceeding the plasma treatment border line may result in surface melting and reformulation. Its noteworthy that most polymers are characterized as non-polar chemically inert low energy surfaces, making them non-receptive to bonding, coatings, and adhesives. Increasing the surface roughness improves wettability and bonding strength for the polymer, which counts advantageous for the end use process.

EDS analysis In order to contemplate on changes in the surface composition before and after plasma treatment, energy dispersive X-ray spectroscopy analyses were conducted for the examined polymeric specimens. Table 4 illustrates the recorded graphs for the examined specimens of polystyrene. Other examined polymers EDS spectra’s are shown in Table S2 (supporting information). Our concern from the EDS analysis is the mass and atom percentages counted for the main components (carbon and oxygen) after plasma treatment which gives an insight on changes in the surface polarity. Table 5 summarizes the changes in the mass and atom percentages occurred in the polymer composition after 2 minutes treatment. Other constituents (e.g Pt and Ti), appeared in the spectral graphs, were excluded from the summary and discussion, as they are considered impurities. Important observation is the increase in the oxygen mass and atomic percentages, for all examined specimens, after plasma treatment. On the other hand, an increase in the carbon mass and atomic percentages was observed for the treated polypropylene and polyethylene. In consequence, the O/C mass ratio 8 ACS Paragon Plus Environment

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has increased for polystyrene (0.016% to 0.2%), HDPE (0.06% to 0.09%) and very slight increase in the case of LLDPE. In contrast, a decrease in the O/C mass ratio from 0.2% to 0.1% is observed for polypropylene. Important reason for the changes in the polymeric composition is the interaction of active species in the plasma with the polymer surface. All species of plasma (electrons, ions ,radicals and UV light) are powerful and hence play a significant role in the surface modification. The constituents of plasma are active enough to break C-C and C-H bonds at the layer of surface to form radicals that produce a variety of oxygen functionalities on the surface via subsequent radical oxidation chemistry. The thickness of the surface layer under plasma effect was measured for several polymeric materials through X-ray photoelectron spectrometry analysis and other analytical methods and found in the range 6-500 nm 38. Since the plasma treatment was performed at atmospheric pressure, some of the oxygen absorbed on the polymer surface when ionized by argon plasma was responsible for the surface oxidation. Moreover, oxygen functionalities on the surface may be formed due to the interaction of plasma radicals with the atmospheric oxygen. As a result, an increase in the carbonyl (C=O) functional group on the polymer surface has occurred and logically leads to an increase in the surface wettability.



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Table 4 EDS analysis spectra’s for untreated and 2 minutes treated specimens of polystyrene Polystyrene a- before treatment

b- treated for 2 minutes


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Table 5 Changes in the mass and atom percentages for oxygen and carbon constituents before and after 2 minutes plasma treatment Mass% (C)

Mass% (O)

Atom%(C)

Atom%(O)

Untreated Polypropylene

11.60

2.35

61.49

9.34

Polypropylene treated for 2 mins.

30.73

3.19

82.22

6.41

Untreated Polystyrene

81.69

1.38

97.52

1.24

Polystyrene treated for 2 mins.

19.28

4.46

69.96

12.14

Untreated Polyethylene

5.28

4.26

37.68

22.85

Polyethylene treated for 2 mins.

8.53

7.23

44.32

28.20

Untreated High density polyethylene

35.51

2.19

86.63

4.01

High density polyethylene treated for 2mins.

26.56

2.43

80.26

5.51

MFI test results A melt mass flow rate test was conducted for untreated HDPE specimen and three HDPE specimens treated by FJPT for 5, 10 and 15 minutes. We aimed at exploring the variations on the polymer performance toward the end use process. Table 6 summarizes the average results of three replicates MFI test. A considerable increase in the MFI was occurred up to 5 minutes treatment. Nonetheless, MFI has reduced sharply for specimens treated for 10 and 15 minutes, and this result could be attributed to the melting of the polymer surface which was happened due gradual increase in the surface temperature. This was confirmed through measuring the mass of the examined polymeric granule before and after several plasma treatment durations. Table 7 illustrates the averages of 4 replicates for the measured mass. It can be observed that negligible reduction in the mass was occurred for HDPE granule treated for 5 minutes. However, higher reductions in the mass occurred for HDPE granule treated for longer periods, which indicates a considerable etching occurred on the polymeric surface. Hence, 5 minutes treatment duration was concluded as the optimum for HDPE pressure pipe grade. 11 ACS Paragon Plus Environment


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Table 6 MFI test results – HDPE specimens

Untreated 0.2186

Melt flow index (MFI) - gm/10 minutes extrusion Treated for 5 Treated for 10 Treated for 15 minutes minutes minutes 0.3145 0.1943 0.1758

Table 7 Measured mass for HDPE granule before and after several durations plasma treatment

Untreated

0.1375

HDPE granule mass (gm) Treated for 5 Treated for 10 minutes minutes

0.135

0.125

Treated for 15 minutes

0.125

Hydrophobic recovery investigation As mentioned in the introduction section, the phenomenon of hydrophobic recovery was considered crucial for polymers treated by plasma, as the features gained on the surface is lost with time. In this research, the hydrophobic recovery action was evaluated through measuring the melt flow index for specimens of HDPE (treated for 5 minutes) within several durations after treatment. It’s worth mentioning that a very limited decrease (< 3%) was observed in the measured MFI along three days after treatment, which is considered beneficial for the polymer end use.

Conclusions Application of flying jet plasma torch was proved effective for remediation of 4 examined polymers. SEM, AFM, EDS analysis have indicated clear modifications in the surface topography and chemistry. As a result of optimized plasma treatment, an increase in the roughness of the polymer has led to change the surface status from hydrophobic to hydrophilic which is counted advantageous for the polymeric outer interactions. However, exceeding 5 minutes treatment, for HDPE specimen, has led to a decrease in the surface roughness due to melting of the surface top layers. MFI test has indicated enhancement in the machinability of HDPE polymer towards the end use process. Measurements of melt flow index, for treated HDPE specimens, along three days after treatment have declared very limited decrease in the measured MFI, which is considered beneficial for the polymeric end use. Hence, it is recommended to apply FJPT treatment for the polymer in its raw shape (granules or powder) before further end use processing. Future work is planned to extend the FJPT application for more polymers. Focus will be given to report surface characteristics of the studied materials, especially changes in the apparent contact angle due to the treatment and changes in the kinetics due to the hydrophobic recovery. 12 ACS Paragon Plus Environment

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Acknowledgments Wameath Abdul-Majeed thanks AL Hilal Polyethylene Pipe Industry Co. Ltd. and Mr. Dhia Omran for conducting the MFI tests and AFM analysis, respectively. The authors thanks Mr. Ahmed Mohammed Said Al-Abri for help in setting up the FJPT experiments.

Supporting Information Please refer to the supporting information file for: 1-SEM images for polypropylene, low density and high density polyethylene’s before and after 2 minutes plasma treatment. 2- EDS spectra’s for polypropylene, low density and high density polyethylene’s before and after 2 minutes plasma treatment

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